[CLI]: `dab update` option to enable/disable multiple-mutations

[severussundar]

To simplify usage, is it possible to have:

"nested-mutations" : true
to imply any supported nested mutations are enabled. Could be a separate PR.

Originally posted by @Aniruddh25 in #1983 (comment)

Introduce a new option to enable/disable all nested mutation operations through CLI.

The resulting JSON in the config file would be of the form

     "runtime":{
      ...
       "graphql": {
           ...
           "multiple-mutations": true/false
       } 
     }

Note: The exact name of the field - multiple-mutations might change as the new name for the feature is under discussion. Will update this issue later with the final name of the field.